The hottest planet


Of the over 200 known extrasolar planets, just 14 pass in front of and behind their parent stars as seen from Earth. This fortuitous geometry allows direct determination of many planetary properties1. Previous reports of planetary thermal emission2,3,4,5 give fluxes that are roughly consistent with predictions based on thermal equilibrium with the planets’ received radiation, assuming a Bond albedo of 0.3. Here we report direct detection of thermal emission from the smallest known transiting planet, HD 149026b, that indicates a brightness temperature (an expression of flux) of 2,300 ± 200 K at 8 µm. The planet’s predicted temperature for uniform, spherical, blackbody emission and zero albedo (unprecedented for planets) is 1,741 K. As models with non-zero albedo are cooler, this essentially eliminates uniform blackbody models, and may also require an albedo lower than any measured for a planet, very strong 8 µm emission, strong temporal variability, or a heat source other than stellar radiation. On the other hand, an instantaneous re-emission blackbody model, in which each patch of surface area instantly re-emits all received light, matches the data. This planet is known6,7,8,9 to be enriched in heavy elements, which may give rise to novel atmospheric properties yet to be investigated.

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Figure 1: Comparison of exoplanetary brightness T b versus equilibrium temperatures T eq for a Bond albedo (wavelength-independent reflectivity) A of 0.3 and uniform planetary emission.
Figure 2: Unprocessed photometric light curve for the 8 µm secondary eclipse of HD 149026b.
Figure 3: Binned light curves, corrected for instrumental effects (see Supplementary Information ).
Figure 4: Equilibrium temperature parameters.


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We thank Spitzer’s Director for discretionary time; G. Squires, and the Spitzer staff for rapid proposal handling and scheduling; B. Hansen, C. Lisse, T. Loredo, and W. T. Reach for discussions; and A. Wolf, J. Winn, G. Henry, M. Holman, H. Knutson and D. Charbonneau for discussions and for sharing results before publication. W. Bowman assisted in preparing Fig. 1. We thank C. Markwardt, the Free Software Foundation, W. Landsman, other contributors to the Interactive Data Language Astronomy Library, and the open-source community for software. This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. This material is based upon work supported by the US National Science Foundation and by the US National Aeronautics and Space Administration through an award issued by JPL/Caltech.

The original data are available from the Spitzer Space Telescope archive, program 254

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Correspondence to Joseph Harrington.

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The original data are available from the Spitzer Space Telescope archive, program 254. Reprints and permissions information is available at The authors declare no competing financial interests.

Supplementary information

Supplementary Information 1

This file contains Supplementary Methods, which inform the analysis used in the paper, including Supplementary Figures 1-7, Supplementary Table 1 and additional references. (PDF 18820 kb)

Supplementary 2

This file contains the light curve and ancillary information analyzed in the article. It is in the standard ASCII Flexible Image Transport System (FITS) format used by astronomers. Its header describes the internal arrangement of data. (ZIP 4501 kb)

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Harrington, J., Luszcz, S., Seager, S. et al. The hottest planet. Nature 447, 691–693 (2007).

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